1. Field of the Invention
The present invention relates to a semiconductor optical integrated element and a method for manufacturing the same made up of a laser diode and a modulator integrated on a substrate.
2. Background Art
In recent years, semiconductor optical integrated elements made up of a laser diode and a modulator integrated on a substrate are used. Examples of adopted waveguide structures for a laser diode or modulator include an embedded waveguide in which both sides of a core layer are embedded with semiconductor, a high-mesa ridge waveguide in which both sides of a core layer are not embedded with semiconductor, and a low-mesa ridge waveguide whose groove does not reach a core layer (e.g., see Japanese Patent Laid-Open No. 2008-10484, Japanese Patent Laid-Open No. 6-260727, Japanese Patent Laid-Open No. 2007-227504 and C. Rolland et al., “InGaAsP-based Mach-Zehnder modulators for high speed transmission systems”, Proceeding of OFC'98 ThH1 (1998)”).
In conventional semiconductor optical integrated elements, the laser diode waveguide and the modulator waveguide have the same structure. When both waveguides are embedded waveguides, the laser diode demonstrates excellent heat radiation characteristics and good high output characteristics, whereas since the modulator has large light propagation loss and a large capacity, high output characteristics and high-speed modulation characteristics thereof deteriorate. When both waveguides are low-mesa ridge waveguides, heat radiation characteristics and high output characteristics deteriorate in the laser diode compared to the embedded waveguides, whereas in the modulator, the capacity increases and high-speed modulation characteristics thereof deteriorate. When both waveguides are high-mesa ridge waveguides, heat radiation characteristics in the laser diode significantly deteriorate compared to those of the embedded waveguides and high output characteristics cannot be obtained, whereas good high-speed modulation characteristics are obtained in the modulator because its capacity is small.
As such, there is no waveguide that satisfies both high output characteristics of the laser diode and high-speed modulation characteristics of the modulator. Thus, it is conceivable to use waveguides of different structures for the laser diode and the modulator. A tapered connection waveguide is proposed which connects waveguides of different structures (e.g., see Japanese Patent Laid-Open No. 2007-227504, Japanese Patent Laid-Open No. 7-74396, Japanese Patent Laid-Open No. 8-78792 and Japanese Patent Laid-Open No. 2000-193921).